1. Field of the Invention
The present invention relates to apparatus used for the thermal development of photothermographic media. In particular, the present invention relates to a filter for use in such thermal development apparatus.
2. Background of the Invention
Thermographic and photothermographic imaging systems based on the generation of silver images by the thermally induced reduction of silver salts are well known in the art. A silver image is generated by the localized (imagewise) reduction of a silver salt, typically an organic silver salt with little or no light sensitivity (referred to as a light insensitive silver salt), by a reducing agent for silver ion. In a thermographic system, the differentiation between the image and the background is controlled by imagewise distribution of heat, with the silver image being formed where heat is applied. In a photothermographic system, a light sensitive silver salt (i.e., silver halide) is placed in catalytic proximity to the light insensitive silver salt. When actinic radiation strike the silver halide, which is sensitive or has been spectrally sensitized to radiation of that wavelength, metallic silver (unoxidized silver, Ag.degree.) is photolytically formed. The photolytically formed silver acts as a catalyst for the further reduction of silver salt, including the light insensitive silver salt in catalytic proximity to the silver halide. Upon heating of the radiation-exposed photothermographic element, the light insensitive silver salts, which are in catalytic proximity to exposed silver halide having photolytically formed silver specks, are more rapidly reduced by reducing agent than are the light insensitive silver salts further from the exposed silver halide. This causes the silver image to be primarily formed where the photothermographic element was irradiated.
The most common type of photothermographic element which is commercially available comprises a silver halide as the light sensitive silver salt (either as in situ formed silver halide or preformed silver halide), a silver salt of an organic acid (usually a salt of a long chain fatty acid (e.g., having carbon lengths of 14 to 30 carbon atoms, such as behenic acid) as the light insensitive silver salt, a photographic silver halide developer or other weak reducing agent as the reducing agent for silver ion, and a binder to hold the active ingredients together in one or two layers (e.g., U.S. Pat. No. 3,457,075).
Development usually occurs by placing the exposed photothermographic element in contact with a heated surface (e.g., a heated roller or platen) or in an inert heated fluid bath. The heated rollers used in the past have generally been fairly open to the environment which has enabled any innocuous materials generated or evaporated by the heating step to escape to the atmosphere. Newer types of imaging systems sometimes are often used in closed work areas or are completely closed systems which do not have ready venting to the atmosphere. Requiring a dedicated venting or exhausting system for these thermal developing units would be burdensome on the users.
Commercial models of thermal processors for photothermographic elements, such as the 3M Model 259B Continuous Thermal Processor, have contained some filtering means on the equipment. In that processor, the filtering means is separated from the actual thermal development area of the processor as shown in the Illustrated Parts Manual for that processor. This filter acts to capture airborne condensate formed from material evaporated from the thermally developed media.
The inventors have found that during thermal development of photothermographic elements in a closed imaging unit certain harmless materials that evaporate during the thermal development step form deposit on the interior of the unit. This condensation of materials (such as the free fatty acid generated upon reduction of the silver salt and then evaporated during development) can adversely affect many aspects of the imaging process. The condensation may clog vents and cause the developer unit to overheat. The condensate may deposit on the heating element and cause localized insulation of the heated surface in a random fashion, producing image variations across the imaged element. Deposits on the pressure rollers can also lend to image variation from differential heating or can cause marking (pressure marking or transfer deposition) on the film. Electronic components can fail due to corrosion when exposed to released vapors. The condensate may deposit on or be transferred to imaging media or seams of the unit. The deposits cause an unsightly appearance and may leave greasy materials on the hands of anyone using the unit. These problems made finding a means of removing the evaporated materials from the vent stream without the need of a dedicated vent (e.g., a vent that accesses the exterior of a room or building or a special ducted vent stream within a building) a necessity.
Copending application Ser. No. 08/239,888 discloses a filter system for use with a photothermographic developing apparatus. Due to damage of filter materials by the relatively high temperatures of the exhaust materials, irregular rates of deposition of condensate in the filter causing channelling, heating of the filter material which prevented continuous deposition of the evaporate, and desirability of moldability, only bonded absorbent particulate filter media, particularly bonded carbon was deemed acceptable. The absorbent particulate filter media serves as the substrate for condensation as well as the absorbing substrate for odor causing by-products. The photothermographic imaging/developing apparatus preferably vents from at least two locations in the imager/developer. The application indicates a preference for locating the filter system within the housing of the developing apparatus and shows a filter system located above the heating element of the developing unit.